ISSUE395|COMPUTER SHOPPER|JANUARY2021 87
c
wouldstartbylookingforthemostcommon
letterinsometextandassumingthatitwasE.
Similarrulescanhelptobreakotherletters.
TheEnigmamachinewasdifferentand
farmorecunning,thankstoasetofelectro-
mechanicalrotors.Thesethreerotorseachhad
26 positionsthatcouldbemanuallyset,with
eachpositionlabelledalphabeticallyfromA
toZonthealphabetring,alsoknownasthe
Ringstellung.Eachpositionalteredtheelectrical
currentthrough the machine, producing a
differentoutcomefor eachposition.
Each keypress moved therotors,changing
the substitution cypherwitheach character
typed.Thefirst rotor moved oncefor each key
press: when anotch onthefirst rotor hit apawl
on thesecond rotor,thatwould movetoo –abit
like theodometer on acar. Anotch on the second
rotorwould hitapawlonthe third rotor, andit
would move as well.Sohitting Amightget you
Dwith thefirst key press, but thesecondtime
you might get R. Consequently, itwas impossible
to use purely linguistic techniquesto break a
code. To decode amessage on an Enigma
machine, the operator needed to know the
original machine’s initial rotorpositioning, which
would be described by alphabetical listing,such
as ‘ADR’, for example.
To make things even more complicated, each
rotor was interchangeable, so it could be placed
in either of the rotor slots. You hadtoknow
whichrotor went where, as wellastheir initial
position. Towardsthe end of the war Enigma
machines had five rotors,with threechosenat
any one time. In addition, thealphabetring could
be rotated, so you’d have to know its position
before you couldset the initial rotor position –
otherwise,the notches and pawls would be out
of sync and the decryption wouldn’twork.
The plugboardatthe front, called the
Steckerbrett, allowed up to sixplugs to be
connected, switching twoletters. ‘Steckering’ A
and Umeantthat typing the letter Awould send
the electrical impulse forU,and vice versa. To
decipher amessage, you had to know the rotor
order, which rotors werebeing used, theirinitial
position and the connections on theplugboard.
Clearly,the Enigmamachine produced code
that was incredibly hardtobreak, withoverone
trillion possible combinations. With everything
encrypted this way, theGermanswereconvinced
that their communications would be safe from
Alliedeyes–fortunately, thiswasn’t the case.
THE POLISH INFLUENCE
BletchleyPark’s success wouldn’t have been
possible if it hadn’t been for the work of the
Polishmathematicianswho assisted the
endeavoursoftheBletchleyParkcode-breakers.
On1stSeptember1932,cryptologistsMarian
Rejewski,HenrykZygalskiandJerzyRózyckiwere
giventhetaskofsolvingthelogicalstructureof
theGermanmilitary’sEnigmamachine.The
needwasimmenseasPolandwasinadesperate
position andcouldbeattackedatanytime.
Themathematiciansgotanearlybreak
whentheyweregiventhelistofcypherkeys
(Enigmasettings)forthemonthsofSeptember
andOctober. Goingback throughintercepted
communicationsforthisperiod,theyworkedout
howtheEnigmawaswiredinternally,andhow
therotorsinteracted witheachother.
Nextcameatheme thatwouldbecome
commonincrackingEnigma:exploitinghuman
error,whichisoftentheweakestlinkinany
securitysystem.ThePolesstarted by usinga
flawinthewaythatthemessageindicatorwas
set.Thisindicatorenabledeverytransmissionto
useadifferentinitialrotor setting.Itworkedin
additiontothedailykey,whichwassetoutin
codebooks.Thisdailycodecontainedtherotor
orderandrotorstobeused,theplugboard
connectionsand,crucially,auniversalinitial
rotorsetting(thegroundsetting).
Usingthismethod,Enigmaoperatorswould
createarandominitialrotorsetting(suchas
‘ABZ’)asthemessageindicator.Theywoulduse
thegroundsettingofthedaytoencrypttheir
newmessageindicator,whichwouldget
transmittedtwicetoensure correctdelivery.
Inourexample,‘ABZ’couldbetransmittedas
‘HRDTSY’.Themessageindicatorwouldthenbe
usedtoencodetherealmessage,whichwould
thenbetransmitted.
DOUBLETROUBLE
This method gave the Polish cryptologists the
informationthatwhenthe message indicator
was sent, the first and fourth letters (H and T),
second and fifth letters (R andS), and thirdand
sixth (D andY)werethe sameletters.Bycoming
up with tablesthat exploited these relationships,
it was possible to reduce the number of possible
combinations down to 105,456. Although the
codes were originalcrackedmanually by using
perforated sheets,Marian Rejewski come up
with an electro-mechanical device (the boma
kryptologiczna), whichwould try every single
one of 105,456 possiblerotor combinations –
knownasa‘brute-force attack’.
There could be several ‘right’ answers, but
another mistake in Enigma made it easyto
identify thecorrect one:alettercouldnot have
itself as its cypher text. In otherwords, if you
typed A, you would never get Alighting up on
the lampboard.Thereforethe cyphertextcould
nothavelettersinthesamepositiononthe
plain-textmessage.Onceonemessagehadbeen
decoded,thegroundsettingofthedaywas
determinedandallmessagescouldbedecrypted.
THEBRITISHTAKEOVER
However,timewasrunningoutforthePolish.
On15thMarch1939,theGermanstook
BohemiaandMoravia,andwithdrewfromthe
German-PolishNon-AggressionPacton 27 th
April1939.Withahugethreatonthedoorstep,
aconferencewasheldinWarsawonthe26th
July,wherethePolishrevealedtotheBritishand
Frenchthatthey’d‘brokenEnigma’andpledged
togiveeachcountry aworkingreconstructionof
themilitary Engimamachine,alongwithdetails
oftheircode-breakingtechniques.Thiswasvital
informationfortheBritish,whohadthusfar
beenstrugglingtodecodethemessages.
On15thAugust1939,theGovernment
CodeandCypherSchool(GC&CS)moved
secretlyintoBletchleyPark–theirmissionto
decodeEnigmasignals.Theparkwaschosen
forseveralreasons,notleastofwhichwasits
proximitytoOxfordandCambridge(manyof
thecode-breakerswererecruitedfromhere).It
wasonthemainrailwaylinebetweenLondon
andBirmingham,ManchesterandGlasgow,and
thenearbyrepeaterstationatFennyStratford
madeiteasytolaydedicatedcablesfor
telephoneandtelegraph circuits.
An amazing teamofmathematicians was
gatheredatBlethley Park. Early work wascarried
out by DillyKnox and John Jeffries (bothof
whomdied of separateillnesses before the endof
the war) and, most famously of all, Alan Turing.
Turing was to play an influential part in
breaking Enigma. Since the Poleshad found the
flaw in theway that the message indicator was
being transmitted, theGermans had changed
tactics and startedprinting their code books
without the ground setting. Thecryptographic
Station Xwas wherethe first code-breaking
took place, but thetransmitter was
relocatedtoadifferentsite and the code
breakers moved into huts
TheinsideofTuring’sBombeis
incrediblycomplicated
